Assessment of petroleum geology and exploration potential of the marine Mesozoic strata in the Qiangtang Basin

To support ongoing hydrocarbon exploration of the marine Mesozoic strata in the Qiangtang Basin， we conduct surveys and reassessments of the regional petroleum geology. Accordingly， several benchmark sections of the basin’s source rocks and reservoirs are established. The results indicate that the Qiangtang Basin contains three dominant source rock sequences （the Bolila， Bagong， and Quse formations）， two suites of carbonate reservoirs （the limestone karst of the Bolila Formation and the dolomites of the Buqu Formation）， three suites of sandstone reservoirs （the Bagong， Quemocuo， and Xiali formations） dominated by tight-sand reservoirs， and three suites of well-developed regional evaporite cap rocks （the Quemocuo/Quse and Xiali formations）. Analysis of oil and gas shows reveals that the marine Mesozoic strata in the Qiangtang Basin experienced the hydrocarbon generation， migration， and accumulation processes. In this basin， oil-generating strata， reservoirs， and cap rocks are well-matched both temporally and spatially， forming two petroleum systems： the Upper Triassic-Lower Jurassic pre-salt system and the Lower-Middle Jurassic intrasalt system. Three hydrocarbon plays are identified in the basin. The first play consists of the source rocks of the Bolila Formation， the karst reservoirs of the Bolila Formation， and the mudstone cap rocks of the lower Bagong Formation. The second one is composed of the mudstone source rocks of the Bolila-Bagong formations， the sandstone reservoirs of the Quemocuo-Bagong formations， and the evaporite cap rocks of the middle-upper Quemocuo Formation. The third one comprises the source rocks of the Quse Formation， the dolomite reservoirs of the Buqu Formation， the sandstone reservoirs of the Xiali Formation， and the evaporite/mudstone cap rocks of the Xiali Formation. Three types of hydrocarbon resources occur in the Qiangtang Basin： conventional， shale， and tight hydrocarbons. In the Maqu area， play fairways cover an area of about 5 × 103 km2. An assessment of the hydrocarbon generation intensity and hydrocarbon accumulation coefficients （HACs） of source rocks in the Bolila-Bagong formations reveals that the Maqu area has conventional hydrocarbon resources of about 0.5 × 109 t when estimated using a minimum resource abundance of 100 × 103 t/km2. Meanwhile， the Bagong Formation contains shale oil resources of approximately 1 × 109 t. In the Biluocuo-Esima area， play fairways with an area of about 8 × 103 km2 are identified. An assessment of the hydrocarbon generation intensity and HACs of source rocks in the Quse and Bagong formations reveals that this area contains conventional hydrocarbon resources estimated at 0.8 × 109 t based on a minimum resource abundance of 100 × 103 t/km2. In this area， the Bagong and Quse formations are identified as play fairways for shale oil exploration， with shale oil resources of approximately 1.6 × 109 t and about 0.8 × 109 t， respectively. In the Shenglihe area， play fairways span an area of 5 × 103 km2， where the Quse Formation holds shale oil resources of about 1 × 109 t.